This invention relates generally to a connecting strategy for batteries and other high current electrical joints, and more particularly to establishing a high current connection between a high current source and a high current load that are used for vehicular and related transportation applications such that partial assembly operations associated with such connection are avoided.
Lithium-ion and related batteries are being used in automotive and related transportation applications as a way to supplement, in the case of hybrid electric vehicles (HEVs), or supplant, in the case of purely electric vehicles (EVs), conventional internal combustion engines (ICEs). The ability to passively store energy from stationary and portable sources, as well as from recaptured kinetic energy provided by the vehicle and its components, makes such batteries ideal to serve as part of a propulsion system for cars, trucks, buses, motorcycles and related vehicular platforms. The flow of electric current to and from the individual cells (i.e., a single electrochemical unit) is such that when several such cells are combined into successively larger assemblies (such as modules and packs), the current or voltage can be increased to generate the desired power output. In the present context, larger module and pack assemblies are made up of one or more cells joined in series (for increased voltage), parallel (for increased current) or both, and may include additional structure to ensure proper installation and operation of these cells. One common vehicular form of the battery pack is known as a power battery, while another is known as an energy battery.
In one form, the individual cells that make up a battery pack are configured as rectangular (i.e., prismatic) cans that define a rigid outer housing known as a cell case. These types of cells are generally assembled into the power battery pack variant. In another form, the individual cells are housed in a thinner, flexible rectangular pouch that are generally assembled into the energy battery pack variant. Both cell types can be placed in a facing arrangement (much like a deck of cards) along a stacking axis formed by the aligned parallel plate-like surfaces. Positive and negative terminals situated on one edge on the exterior of the housing of each cell are laterally-spaced from one another to act as electrical contacts for connection (via busbar, for example) to an outside load or circuit. With particular regard to the prismatic can, numerous individual alternating positive and negative electrodes are spaced apart from one another within the can along the stacking direction and kept electrically isolated by non-conductive separators. Leads from each of the negative electrodes are gathered together inside the housing to feed the externally-projecting negative terminal, while leads from each of the positive electrodes are likewise gathered together to feed the externally-projecting positive terminal.
In a traditional electrical connection approach for cells in a battery pack, a connecting ring is placed over a stationary stud or related post-like anchoring location that is electrically coupled to one or more connectors that are joined to the respective anode or cathode tabs that extend from each aligned cell within a group of battery cells. The stud and ring act as an electrical joint between the individual cell tabs and the conductive path that is used to deliver the current to the electric motor or related load. During battery pack assembly, electric connection is established immediately upon the placement of the ring over the stud; this situation is known as partial assembly. To complete the connection, a threaded nut or related cap is coupled to the stud to ensure relative permanence of the connection between the mating surfaces formed at the joint.
This partial assembly approach has a tendency to produce a loose bolted connection, which in turn leads to high resistance electrical joints; these tend to be very hard to detect. One factor contributing to the increased joint resistance is a lack of adequate force to promote secure contact between the joined components of the partial assembly. Another is a lack of adequate surface area; this latter shortcoming reduces the size of the flowpath through which the current may pass such that in places where such flowpath is established, which in turn leads to increased localized heating. In some cases, the heat is sufficient to cause degradation of coatings, which further increases joint resistance and ultimately can lead to overheating of the joint, where further damage (such as to adjacent plastic components) may ensue. Extensive quality or diagnostic checks may help reduce the occurrence of such high-resistance joining at the point of manufacture. Such diagnostic techniques (which may require thermal images to be taken during periods of high current flow through the connected region to determine the resistance of the assembled joints) add considerable complexity and related expense to the assembly process. Moreover, there are no known methods to perform such diagnostics at post-manufacturing locations, such as those involved in the repair or related service to the battery cells and packs.
While most of the subsequent discussion is focused on high current joints used for batteries in general and automotive battery applications in particular, the invention disclosed herein may be equally applied to other high current joints, including those used for electric motors, controllers or the like, as well as those used as ancillary battery equipment. For example, a joint that is formed in a battery disconnect unit (BDU) could also benefit from reduced reliance on partial assembly and its attendant difficulties.